During heavy exercise (greater than about 50% VO2MAX) fluid is lost from the vascular compartment due to the establishment of hydrostatic and oncotic gradients between the tissues and the capillary blood. The purpose of this study will be to determine what effect alterations in blood volume and composition have on the circulatory and temperature regulatory control systems during exercise. A minimum of six healthy volunteers will exercise (cycle ergometer) on separate days, for 30 min. (60-70% VO2max) in cool (18 degrees C) and warm (30 degrees C) environments. During each test measurements of esophagealand skin temperature (thermocouples), heart rate (monitor), cardiae output (CO2 rebreathing), local (dew point hygrometry) and total (body weight loss) sweat rate, and peripheral blood flow (nevous occlusion plethysmography) will be obtained. Blood samples will be collected before, during and after exercise and hematocrit, hemoglobin, total protein, osmolality, electrolytes, lactates, catecholamines, and antidiuretic hormone concentrations will be determined. Changes in blood volume will be calculated by comparing hematocrit and hemoglobin values during exercise tests, to values obtained from duplicate determinations of blood volume (51Cr, RISA) obtained at the start of the study. first, body fluid patterns will be determined for each subject in duplicate control tests. Next, experimental exercise tests will use the same protocol as control tests, except that after 10 min. of exercise, a solution will be infused intravenously. The infusion will continue for the remainder of the exercise to maintain blood volume. The three infusates used in separate experiments will be isotonic saline, hypertonic saline and isotopic serum albumin. By this procedure, we will assess the effect of changes in blood volume, electrolyte and protein concentrations on the circulatory and temperature regulatory exercise responses. The knowledge gained by this study will be of particular interest to anyone interested in the responses of human to stress, including physiologists, clinicians and workers in sports medicine and occupational medicine. For myself, this study is a logical extension of a long-term research goal - to understand the interactions and forces involved in body fluid regulation.